The present invention relates to absorber rods for inclusion in nuclear reactor control clusters. It is particularly usable in reactors that are moderated and cooled by pressurized water, where the core is constituted by fuel assemblies, each having a bundle of fuel rods held at the nodes of a regular array by a skeleton assembly formed by nozzles connected by guide tubes carrying rod spacer grids. Under such circumstances, each cluster is constituted by a spider connected to a control mechanism and carrying rods containing absorber material for the purpose of being inserted to a greater or lesser depth in the guide tubes or even for being completely extracted from the core.
Usually, a reactor has its power adjusted and is stopped by using various groups of clusters of different makeups (such as xe2x80x9cblackxe2x80x9d clusters which are highly absorbent, and xe2x80x9cgrayxe2x80x9d clusters which are less absorbent).
xe2x80x9cBlackxe2x80x9d clusters are constituted by rods containing a highly absorbent material, such as an Ag-In-Cd alloy, also known as AIC, or boron carbide B4C, in cladding which is generally made of stainless steel.
Those rods present limitations, in particular when they are for use in reactors operated to xe2x80x9cfollow loadxe2x80x9d and/or for very long use. The Ag-In-Cd alloy is subject to creep and to swelling under irradiation. Boron carbide B4C presents a large amount of swelling under irradiation which means that it cannot be used in the bottom portions of rods which are the portions that are inserted the most often into the core.
It is also known that hafnium absorbs neutrons and does not creep at the working temperatures and does not swell under irradiation. However it needs to be protected from hydriding if it is in steel cladding and it needs to be protected from wear if it is allowed to rub against guide elements. Welding it to the stainless steel used for making the protective cladding of rods containing boron carbide pellets gives rise to connections that are fragile and sensitive to hydriding, as explained below.
In theory, replacing Ag-In-Cd with hafnium in steel cladding makes it possible to avoid the absorber swelling. But the hafnium must not be allowed to hydride.
Attempts at making rods with hafnium in cladding have encountered difficulties. After the natural oxide film that forms on contact with air during manufacture has been worn away by friction, the hafnium absorbs the hydrogen that passes through the steel cladding and it swells, to such an extent that it can become necessary to change clusters prematurely.
A similar problem arises with xe2x80x9cgrayxe2x80x9d cluster rods containing rods that are less absorbent.
Proposals have already been made (French patent application No. 96/07430, which issued as French Patent No. 2,749,968) to avoid the difficulty by using rods having bottom portions constituted by un-clad bars of HfZr or of hafnium, and replacing the stainless steel cladding in the top portion of the rod with cladding made of an Hfzr alloy containing pellets of (HfZr)B2 or of HfZr.
To avoid adopting a cladding material that has mechanical characteristics that are very inferior to those of stainless steel, the invention seeks in particular to provide an absorber rod capable of withstanding irradiation over a long period of time in a reactor and also making it possible to retain stainless steel cladding and a stainless steel top connection plug for connection to a spider.
To this end, the invention provides in particular an absorber rod suitable for use in a control cluster and comprising stainless steel cladding closed by plugs and containing a stack of absorber pellets, the rod being characterized in that it also comprises an end bar of hafnium which is advantageously not clad, and which is secured to the bottom plug of the cladding by a surely mechanical connection. The stack of pellets is usually constituted by boron carbide; it could also be constituted by hafnium and zirconium boride, pure or mixed (as described in French patent application No. 96/07430) or indeed of rare earth oxides (europium, dysprosium), pure or mixed with other oxides.
The solid or hollow bar generally constitutes at least 15% of the length of the rod, i.e. of the distance over which the rod can be moved by its mechanism. Frequently, the bar constitutes about 25% of the total length of the rod. It is the bar which is inserted most frequently into the core.
The non-clad hafnium bar comes into direct contact with the pressurized water, it does not hydride, it does not swell, and it does not creep. However, in this use, hafnium that is not clad needs to be protected:
against wear, for the portion thereof that comes into contact with its neighbours; and
against hydriding at the connection between the hafnium and the stainless steel.
The best protection is a layer of oxide which is established on an underlying diffusion layer that is impermeable to hydrogen and that withstands wear. This layer forms naturally in the medium that is to be found in pressurized water reactors (pressure about 150 bars and temperature in the range 280xc2x0 C. to 350xc2x0 C.) but only to a thickness that is generally not sufficient to provide effective protection against wear. This layer can also be created, prior to mounting the rod in the reactor, by applying surface treatment such as that described, for example, in document EP-A-0 421 868. In some cases, the thickness still runs the risk of being too little to withstand the highest amounts of wear.
An initial protective layer against wear can also be created by controlled oxidation of the outside surface in an atmosphere of pure oxygen or of oxygen and argon, at a temperature lying in the range 800xc2x0 C. to 950xc2x0 C. The layer is advantageously at least 5 micrometers (xcexcm) thick initially (as described in French patent application No. 96/07430).
Rubbing tests performed in pressurized water have shown that it is desirable to have oxide to a thickness of at least 5 xcexcm to 10 xcexcm in order to withstand wear. However, if the thickness of oxide formed by thermal oxidation is too great, then stresses at the metal/oxide interface are high and can lead to the oxide layer spalling off. Nevertheless, even if the oxide does spall off, resistance to wear continues to be provided by the underlying diffusion layer providing it is of sufficient thickness (of the order 12 xcexcm to 15 xcexcm).
The oxidizing process advantageously establishes an oxide layer that is sufficiently thick to prevent hydriding (in particular in the zones connected to austenitic stainless steel) and forms a diffused layer that is quite thick and withstands stresses associated with wear.
An oxide layer that is too thick, exceeding 10 xcexcm, runs the risk of spalling. A good compromise can be achieved by aiming for an oxide layer that is 5 xcexcm to 10 xcexcm thick and a diffusion layer having a depth of 15 xcexcm to 50 xcexcm, and in particular of 25 xcexcm to 30 xcexcm. There is generally no point in attempting to obtain a thicker diffused layer, given the wear stresses that are encountered by absorber rods in pressurized water nuclear reactors.
A compromise can be found by limiting the treatment temperature so as to leave time for diffusion to take place and thus for internal stresses to decrease, and by limiting the rate at which oxygen is supplied either by limiting its partial pressure during oxidization by acting on the total pressure and on its dilution, or by interleaving diffusion stages between the oxidizing stages. The flow speed of the oxidizing gas is another parameter of the process.
Thus, to achieve a density per unit area of included oxygen lying in the range 0.0001 liters per square centimeter (l/cm2) of substrate to 0.01 l/cm2 of substrate, oxidation performed at 860xc2x110xc2x0 C. for 6 hours at a total pressure in the range 0.1 millibars (mbar) to 0.7 mbar in an argon atmosphere containing 3% to 25% oxygen makes it possible to obtain, on hafnium containing 300 parts per million (ppm) of iron and 300 ppm of oxygen, an oxide thickness of 6xc2x11 xcexcm and a diffused layer to a depth of 35 xcexcm to 50 xcexcm.
A similar result can be obtained by thermal oxidation for 6 hours at 920xc2x110xc2x0 C. in an argon atmosphere containing 3% oxygen and acting on the duration of the oxidation and diffusion stages, e.g. by repeating the following cycle six times:
oxidation for 1 minute; and
diffusion under pure argon for 1 hour.
More generally, it is possible to make a protective layer by controlled oxidation of the hafnium over a period of 3 hours to 12 hours at a temperature in the range 820xc2x0 C. to 950xc2x0 C. using oxygen diluted to a concentration of 1% to 50% under low pressure (a few fractions of a millibar); it is also possible to use oxidation-diffusion treatment in which the duration of the oxidation lies in the range 0.1% to 10% of the total time of the treatment.
Since stainless steel is permeable to atomic hydrogen, it is necessary to preserve the hydrogen-impermeable layer of oxide over the connection between the bottom plug of the cladding and the bar and/or to allow for said oxide layer to be renewed on contact with water. Most conventional joining methods do not allow this condition to be satisfied and/or lead to a connection between the hafnium and the steel that is fragile and incompatible with the alternating stresses that are to be encountered during reactor operation.
For example, thermally welding hafnium directly to steel gives rise to intermetallic phases which make the connection fragile, and in addition welding gives rise to continuity of material through which hydrogen can migrate from the stainless steel to the hafnium. Solid state welding, e.g. by diffusion or by friction, leads to connections that are too fragile and also leads to continuity of material that gives rise to hydriding.
Purely mechanical connections make it possible to avoid the above drawbacks. Suitable methods include the following in particular:
cold isostatic compression assembly making it possible to obtain crimped connections having good mechanical characteristics but without creating continuity of material; and
screw assembly where the bar is screwed into the plug, with the threaded connection being prevented from rotating by being deformed.
It is also possible to consider assembly by magnetostriction or by explosion, thereby providing a crimped connection that is extremely robust. Nevertheless, this solution suffers from the drawback of requiring the presence of an aluminum alloy thruster, given the insufficient electrical conductivity of stainless steel and hafnium. The thruster remains connected to the part, and removing it mechanically or by caustic soda etching runs the risk of damaging the connection.
It is also possible to envisage assembly by interfitting the two components that are to be connected together, boring a hole, and inserting a pin. Nevertheless, that solution is mechanically weaker than using a braked screw connection.
The invention also provides a method of assembly by cold isostatic compression which can be used not only for making an absorber rod, but also whenever it is desirable to make a connection between a part made of hafnium or a material presenting similar characteristics (such as zirconium and titanium) and a part made of stainless steel, of nickel alloy, or any other alloy that cannot be welded to materials of the hafnium class, one of the two parts being formed in the portion that is to be mounted, as a tube which receives the other part, which is in the form of a solid cylinder.